The invention relates to sewing machines for producing various stitch patterns including a pattern of straight stitches, and more particularly relates to a thread tightening device for such sewing machines, which are operated to provide a most suitable formation of stitches (the interlocking conditions of the upper and lower threads) in accordance with the patterns to be selected.
Heretofore the thread tension has been generally implemented only with respect to the upper thread because it is easy. The lock stitching sewing machines of this kind are generally provided with a thread tension adjusting device with an operating dial which is operated to adjust the upper thread in accordance to the selected patterns. The lower thread tension is sometimes adjusted by manual adjustment of the bobbin case holding the lower thread.
With respect to the pattern stitching sewing machines, it is necessary to make an adjustment of thread tension in order to provide a most suitable interlocking condition of the upper and lower threads in accordance to the different patterns to be stitched. For the sake of simplicity of explanation reference should be made to FIGS. 1A, 1B and 2A, 2B of the drawings. For example, with respect to the straight stitches as shown in FIG. 1 (A) and (B) the interlocking point "a" of the upper thread 1 and to lower thread 2 is preferable to be positioned about at the center in the thickness of the fabric. On the other hand, with respect to the zigzag stitches as shown in FIG. 2 (A) and (B), the interlocking point "d" is preferable to be positioned at the lower side in the thickness of the fabric as shown in FIG. 2 (B), because the interlocking point "c" positioned at the center in the thickness of fabric in FIG. 2 (A) becames visible at the upper face of the fabric and is undesirable in the pattern stitches. Such a position of interlocking point with respect to the fabric is determined by the relation between the upper and lower thread tensions. Therefore, in order to stitch different patterns in a best condition, it becomes necessary to adjust the tension of the upper thread as well as of the lower thread to properly meet these different patterns. In general, however, the machine operator adjusts only the upper thread for different patterns by means of the upper thread tensioning device, and scarcely adjusts the lower thread tension, because the adjustment of lower thread tension is very difficult and time-consuming.